1. Field of the Invention
This invention relates to a laser optical system provided with a laser beam source. This invention particularly relates to a laser optical system wherein no side lobe is generated around a convergence spot when a laser beam emitted by the laser beam source is converged.
2. Description of the Prior Art
Laser optical systems have heretofore been widely used as, for example, a means for generating a scanning light beam in scanning recording apparatuses and scanning read-out apparatuses. Among laser optical systems, laser optical systems using a semiconductor laser as the laser beam source are advantageous in that the semiconductor laser is smaller, cheaper and consumes less power than a gas laser or the like, and in that direct analog modulation wherein the optical output is changed by control of the drive current can be effected,. Therefore direct modulation may be carried out by the use of signals generated in accordance with image information in the case where the laser optical system is used in a scanning recording apparatus.
In the aforesaid laser optical system, a beam diameter adjusting plate having an aperture through which only the laser beam portion at the center of the laser beam emitted by the laser beam source is to be passed is often disposed in the optical path of the laser beam. The beam diameter adjusting plate is used for various purposes, for example for increasing the focal depth at the time the laser beam is converged, as disclosed in Japanese Patent Publication No. 58(1983)-20015. Also, in the case where a semiconductor laser is used as the laser beam source, the beam diameter adjusting plate is often disposed so that the light in a low output region of the laser beam emitted by the semiconductor laser can also be utilized without increasing the diameter of the convergence spot at the convergence position. The relationship between the output of the semiconductor laser and the diameter of the convergence spot and the effects of the beam diameter adjusting plate will be described hereinbelow.
It has heretofore been known that laser light emitted by a semiconductor laser may be classified into laser oscillation light and light in the natural light emission region. FIG. 8 shows the relationships between the drive current for the semiconductor laser on one hand and the laser oscillation light and the light in the natural light emission range on the other hand. In FIG. 8, line "a" indicates the relationship between the drive current and output of the light in the natural light emission region (hereinafter referred to as natural emission light), and line "b" indicates the relationship between the drive current and the output of the laser oscillation light. As shown, in the case where a drive current is applied to the semiconductor laser, no laser oscillation light is emitted and only the natural emission light is emitted until the level of the drive current exceeds a threshold current Io. As the level of the drive current increases, the output level of the natural emission light increases gradually. However, the laser oscillation light starts being generated when the level of the drive current exceeds the threshold current Io, and the output of the laser oscillation light becomes large thereafter. At this time, the ratio of the output of the natural emission light to the output level of the laser beam as a whole diminishes, and substantially laser oscillation light alone is emitted. The relationship between the level of the drive current and total amount of natural emission light and the laser oscillation light generated by the semiconductor laser is indicated by curve "c".
The natural emission light contains various angle components as compared with the laser oscillation light, and therefore the natural emission light cannot be converged to a spot diameter as small as the spot diameter of the laser oscillation light. Therefore, in the case where the semiconductor laser optical system is used in a scanning recording apparatus wherein the recording light must be modulated over a wide dynamic range and even light beams of the low output region in which the natural emission light is predominant must be utilized, the beam diameter in the low output level region increases and the spatial resolution of scanning deteriorates. Accordingly, in Japanese Patent Application No. 61(1986)-196352, the applicant proposed a semiconductor laser optical system provided with a beam diameter adjusting plate having an aperture through which only the center portion of the laser beam is to be passed, on the basis of the fining that the diameter of the convergence spot can be reduced for natural emission light as well when the beam diameter adjusting plate is disposed in the optical path of the laser beam.
As mentioned above, the beam diameter adjusting plate is used for increasing the focal depth, or, in the case of the semiconductor laser, for preventing the diameter of the convergence spot of the natural emission light from increasing. In the case where a lens such as a collimator lens disposed in the optical path has a diameter smaller than the diameter of the incident light beam, the lens itself may act in the same manner as an aperture and may substantially constitute the beam diameter adjusting member.
However, in the case where the beam diameter adjusting member mentioned above is disposed in the optical path of the laser beam emitted by the laser beam source, a part of the laser beam is eclipsed by the contour portions of the aperture of the beam diameter adjusting member. As a result, when the laser beam after passage through the beam diameter adjusting member is converged, a weak first-order light component, a weak second-order light component and the like are generated as side lobes around a zero-order light component converged to a predetermined position. When such side lobes are generated, various problems consequently arise, for example image ghosts appear in the case where the laser optical system is used in a scanning recording apparatus and recording of the image information is carried out by use of the converged laser beam.